Tracking assets utilizing Radio Frequency Identification (RFID) technology is well known, similarly it is well known that metal surfaces interfere with the functionality of any RFID tags mounted thereon. This is because the electric field boundary condition requires that the electric field component of the propagating electromagnetic energy evanesce in close proximity to the metallic surface. Thus the fabrication of an effective passive (e.g. powered by the incident electromagnetic field energy) RFID tag for Mount on Metal (MoM) applications presents a technical challenge. Further, it is advantageous for the operation of a RFID tracking system to reduce the overall size of the tag, i.e. its physical footprint on the asset.
Two approaches are known in the art for overcoming the technical hurdle for a MoM tag. The first is to provide a standoff or physical separator between the metallic surface and a RFID tag designed to function in free air, remote from any absorbing or reflecting surfaces. In the absence of the standoff, the tag does not function when mounted on the surface. It is also well known that the selection of standoff materials can significantly influence the performance of the MoM tag. A well-known approach is to use a material that is an efficient absorber of Electro-Magnetic (EM) energy as the spacer. Were the material a perfect absorber the tag would function just as it could in free air, since there would be no interaction of the metallic surface with the field in the vicinity of the tag.
Another approach that has been attempted is the use of a “patch” antenna. In this case the tag antenna incorporates a solid conductive ground plane at a fixed distance from the radiating element of the antenna. There is fixed impedance (through the RFID chip) between the ground plane and the radiating element. The shape and size of the radiating element is manipulated together with the separation between the ground plane and the radiating element. In this case the metallic ground plane serves as a reflector for EM energy; this property also affects the radiation pattern of the tag, significantly increasing the directionality of radiation, away from the ground plane. There have been attempts to utilize the metallic surface as a ground plane; however this is not generally the preferred approach, owing to inconsistency in coupling to the metallic surface and the variability of conductivity of the surface itself (copper vs. aluminum vs. steel) which may vary widely by material.
Both approaches are valid in terms of improving the performance of the tag in proximity to a metal surface; however, neither adequately addresses the issue of size reduction of the tag. For the patch antenna, it is possible to decrease the size of the radiating surface by using a ceramic material with a high dielectric constant as the spacer between the ground plane and the radiating surface. However tags of this design are expensive to produce, owing to the high cost of the ceramic material relative to polymeric or epoxy-glass composite materials used in circuit boards.